1. Field of the Invention
The present invention relates to new conductive compositions, elements and processes. More specifically, the conductive compositions of the present invention are dispersions of hydrophobic latex polymer particles having associated therewith polyaniline acid addition salt semiconductors. Processes for making polyaniline saltcontaining latices and methods for preparing elements having coatings of the compositions form other aspects of the present invention.
2. Description Relevant to the Prior Art
The unwanted buildup of static electricity on an insulating support has been a continuing problem. It is well-known that a thin conductive layer will prevent static buildup but, while it is possible to formulate a conductive composition that can be coated on a support, it has been quite difficult to combine these conductive properties with other desirable physical properties.
The stringent physical and optical requirements for photographic elements make the formulation of suitable antistatic compositions for these elements particularly troublesome. Many conductors are known which can be coated on photographic elements to provide static protection. One particularly useful class of compositions which can be used in photographic elements is a composition containing the polyaniline acid addition salt (hereinafter "polyaniline salt") semiconductor described in U.S. Pat. No. 3,963,498 issued June 15, 1976, to Trevoy. These semiconductors are formed by the reaction of a neutral polyanilineimine (hereinafter "polyaniline") with an acid. These semiconductors offer a number of advantages when used in antistatic coatings, particularly when used with photographic films. For example, because these materials are electronic conductors as opposed to ionic conductors, their conductivity is relatively independent of relative humidity. Thus, they retain high conductivity under conditions of low humidity where the buildup of unwanted static electricity is particularly difficult to control. Further, these semiconductors retain their conductivity when coated in a suitable binder and therefore can be used in a variety of elements using conventional coating techniques. Still another advantage of these semiconductors is that they are relatively inexpensive and therefore can be used on a relatively large scale at low cost.
The polyaniline salt semiconductors of Trevoy offer a number of advantages; however, further improvements have been sought. While coatings containing a relatively low coverage of these semiconductors are useful in reducing the resistivity of an insulating support to a certain extent, relatively high coverages of these semiconductors, when used in conventional coating compositions, are required to achieve sufficient conductivity to eliminate static problems under severe conditions. For example, in order to achieve resistivities on the order of 10.sup.6 ohm/sq, it is necessary to coat the semiconductors of Trevoy at coverages greater than about 35 mg/m.sup.2. Unfortunately, these semiconductors are colored and at these coverages impart to the elements on which they are coated an undesirable density. As an illustration, a coating containing 35 mg/m.sup.2 of a typical semiconductor disclosed in the Trevoy patent, e.g., N-{p-[(4-methoxyanilino)anilino]phenyl}-1,4-benzoquinone imine p-toluenesulfonic acid salt, would have a highly desirable conductivity of about 1.0.times.10.sup.8 ohm/sq, but would also have an integrated optical density of about 0.025 in the visible portion of the spectrum. If the coverage of the polyaniline salt in such a layer were to be reduced so as to reduce the undesirable optical density, the resistivity would increase. For certain critical applications such as, for example, in the production of transparent photographic materials, it would not be possible to get sufficiently high conductivity while at the same time desirable low optical density. It is readily apparent that improvements in the semiconductive coating compositions would be extremely desirable.
Aside from the optical density problems associated with the semiconductors of Trevoy, these semiconductors are, in general, insoluble in water. This can be undesirable because coating layers onto photographic supports is more safely and economically accomplished if water can be used as the basis for the coating composition. Extensive milling permits dispersions of water-insoluble semiconductors to be made in the presence of protective colloids such as gelatin. However, milling in this manner is time-consuming and energy-intensive. It would be highly desirable if a suitable method of coating semiconductors from water could be devised.
It is known to use latex dispersions as binders for conductive materials. In conventional processes such as those described in U.S. Pat. No. 4,011,176, the antistatic materials, such as semiconducting compounds, are simply dispersed in the continuous phase, along with the latex particles. This usually requires extensive mixing and/or milling in order to disperse water-insoluble antistatic material. When this is attempted with polyaniline salt semiconductor antistatic materials, it produces a useful aqueous-based coating composition. However, when the latex is coated and coalesced on a support, high coverages of the polyaniline salt semiconductor are still required to produce the desired high conductivity. This high coverage again results in undesirable density.
While for many reasons semiconductors are highly desirable in photographic elements, the prior art does not suggest a solution to the difficult problems discussed above. There is no suggestion as to how these semidoncutros can be coated from aqueous solutions in order to produce high-conductivity coatings.